10 Frequently Asked Questions about Center Pivots Gulf Coast Irrigation Conference Sinton, November 18, 2008 Guy Fipps Professor and Extension Agricultural Engineer Biological and Agricultural Engineering Texas AgriLife Extension Service

1) 1) How much water do I need? 2) 2) Is my pond large enough to supply my irrigation water requirements? 3) 3) What kind of sprinkler packages are available? 4) 4) What type of water applicator is best? 5) 5) What are the most common design problems seen with center pivots? 6) 6) What size should my mainline pipe be? 7) 7) Do I need pressure regulators? 8) 8) What are the key design considerations for a pivot? 9) 9) How can I check to see if my pivot is operating at design specifications? 10) 10) What are the considerations when renozzling a pivot?

How Much Water Do I Need? Irrigation Systems are designed to supply peak water demand of crops (inches per day, inches per week etc.)

Bulletin 6019 Crop water demand information useful for sizing irrigation systems (shown are values for South Texas) (copy of bulletin are posted at Crop Peak Demand (inches/day) cotton0.23 corn0.32 citrus0.16 sorghum0.22 perennial pasture 0.25 small grains 0.26 vegetables0.16

Example: Pasture/forage in South Texas Peak water demand 0.25 inches/day = 6789 gal/acre/day (note: 1 ac-in = 27,154 gal) Total Gallons Needed per day 10 acres 50 acres 100 acres 120 acres 67,885 (2.5 ac-in) 339,425 (1.04 ac-ft) 678,850 (12.08 ac-ft) 814,600 (2.5 ac-ft)

Example: Pasture/forage in South Texas Peak water demand 0.25 inches/day = 6789 gal/acre/day Pumping rate – % efficiency 10 acres 50 acres 100 acres 120 acres 47 gpm 235 gpm 470 gpm 566 gpm

Example: Pasture/forage in South Texas Peak water demand 0.25 inches/day = 6789 gal/acre/day Pumping rate – 12 80% efficiency 10 acres 50 acres 100 acres 117 gpm 587 gpm 1175 gpm

Pumping rate and irrigation capacity From Table 2 of Center Pivot Irrigation in notebook (assumes irrigating 24 hours per day) From Table 2 of Center Pivot Irrigation in notebook (assumes irrigating 24 hours per day) GPM/acreInches per dayInches per week

Is my pond (tank) large enough to supply my water requirements? Using an on-farm pond (or water storage facility) to provide 0.25 inches/day? Calculate pond size: surface area x average depth = water volume (acres) (feet)(ac-ft) (acres) (feet)(ac-ft)

Example: Pasture/forage in South Texas Is your pond large enough to provide 0.25 inches/day? Water Supply in Weeks ( at 100% irrigation efficiency) 50 acre field 100 acre field 120 acre field 1 7 ac-ft 15 ac-ft 17 ac-ft 2 15 ac-ft 30 ac-ft 35 ac-ft 3 22 ac-ft 45 ac-ft 55 ac-ft

Example: Pasture/forage in South Texas Is your pond large enough to provide 0.25 inches/day? Water Supply in Weeks ( at 80% irrigation efficiency) 50 acre field 100 acre field 120 acre field 1 9 ac-ft 19 ac-ft 21 ac-ft 2 19 ac-ft 38 ac-ft 44 ac-ft 3 28 ac-ft 56 ac-ft 69 ac-ft

type Natural gas ElectricturbineElectriccentrifugalDiesel Cost$1.49$2.00$2.52$3.07 Fuel cost* $11 MCF $0.11 kwh $2.65 gal How much money can I save by reducing my operating pressure? Typical Pumping Costs in Texas: Acre-inch per 100 ft head (or 43 psi) * my fuel costs on Oct 22, 2008

pressure15psi30psi45psi60psi90psi cost (per ac-ft) $10.44$20.76$31.20$41.64$57.72 Costs of Pressurizing Water Per acre-foot of water, electric centrifugal pump at 0.11 kwh

pressure15psi30psi45psi60psi90psi cost (per ac-ft) $1253$2492$3744$4997$6926 Costs of Pressurizing Water For applying 1 ft of water on a 120 acre field: electric centrifugal pump at 0.11 kwh

What types of sprinkler packages are available? (1)high pressure impacts (2)medium elevation spray applicators (MESA) (3)low energy precision applicators (LEPA) (4)low elevation spray applicators (LESA)

Water-move pivot

Older pivot with high pressure impact sprinklers

MESA (medium elevation spray applicators)

Over-pressured MESA system

LEPA (low energy precision applicators) bubble mode

LEPA with alternate row furrow dikes

LESA (low elevation spry applicators)

What types of applicator is best? LEPA and LESA: LEPA and LESA: –have the lowest operating pressures (6 – 10 psi) –the highest efficiencies (90-98%) –But require higher skill level for proper irrigation water management LEPA may require practices to control runoff LEPA may require practices to control runoff

What types of applicator is best? MESA MESA –About 10% of the water applied to high canopy crops is lost to evaporation –Typical efficiencies are 80-90% with proper design

What are the most common design problems seen with center pivots? mainline too small elevation changes in field not considered in the design end gun added system designed for incorrect flow rate

What size should my mainline pipe be? For the same diameter pipe: – –The higher the flow rate, the higher the friction losses Try to keep friction loses to 10 psi if practical Consider telescoping for larger flow rates and for half mile systems

What size should my mainline pipe be? Friction loss in center pivot mainlines (psi) Flow rate, GPM Mainline pipe diameter, inches 66 5/

What size should my mainline pipe be? Telescoping Example: Pivot is 1316 ft long with a flow rate of 1100 GPM   Case 1: all 1316 ft is 6 5/8-inch pipe   Friction loss is 19 psi   Case II: 640 ft of 8-inch and 676 ft of 6 5/8-inch   Friction loss is 10 psi

Do I need pressure regulators?  Depends upon the:  Elevation change in the field (maximum change in elevation from pivot point to highest/lowest point in field  System design pressure (pressure at the nozzle)  Regulators require about 3 psi more than their pressure rating to operate properly  A 6 pound regulator requires an operating pressure of 9 psi

Do I need pressure regulators?   Pivots should be designed to maintain less than 20% variation in system design pressure Table 5. % variation in design pressure Elevation change, ft System design pressure (psi)

What are the key design considerations for a pivot? elevation changes in field considered mainline pipe sized correctly efficient water applicators matched to available water supply matched to water requirements of crop

How can I check to see if my pivot is operating at design specifications? obtain a copy of the pivot design printout note the following: 1) total flow rate 2) pressure at the pivot point 3) pressure at the last nozzle or pressure regulator

How can I check to see if my pivot is operating at design specifications? Do you have enough water (gpm) as measured by your flow meter? Is your flow meter accurate? is the meter installed in a straight section which is at least 15 pipe diameters long?

How can I check to see if my pivot is operating at design specifications? Does the pressure match at the pivot point? Install a pressure gage in the last drop just above the water applicator or pressure regulator Is the pressure equal to or greater than what’s required?

Considerations when renozzling (see Section 15 of Pivot Publication) Actual lowest and highest elevation in field in relationship to the pivot point is used in the computer design printout. Actual measured flow rate and pressure available by pump (or water source) is used in the computer design printout. Friction loss in pivot mainline for quarter- mile long systems is no greater than 10 psi.

Considerations when renozzling (see Section 15 of Pivot Publication) Mainline outlets spaced: Mainline outlets spaced: – a maximum of 60 to 80 inches apart –or alternately, 2 times the crop row spacing. For non-leveled fields: For non-leveled fields: –less than 20% pressure variation in system design operating pressure is maintained when pivot is positioned at highest and lowest point in the field –(computer design printout provided for each case).

Considerations when renozzling (see Section 15 of Pivot Publication) Pressure regulators are evaluated for fields with more than 5 feet elevation change from pad to the highest or the lowest point in the field. Pressure regulators are evaluated for fields with more than 5 feet elevation change from pad to the highest or the lowest point in the field. Copy of pivot design printout provided by dealer. Copy of pivot design printout provided by dealer.

Considerations when renozzling (see Section 15 of Pivot Publication) No end gun. No end gun. Consideration was given to equipping the pivot with either LEPA or LESA applicators Consideration was given to equipping the pivot with either LEPA or LESA applicators

Considerations when renozzling (see Section 15 of Pivot Publication) Propeller flow meter or other type of flow measurement device Propeller flow meter or other type of flow measurement device – with an accuracy of + – with an accuracy of + 3% – –installed in water supply pipeline near pivot point in a straight section 10 pipe diameters upstream and 5 pipe diameters downstream from the flow meter.

Considerations when renozzling (see Section 15 of Pivot Publication) Two pressure gauges Two pressure gauges –one on the mainline near the pivot point, –one in the last drop, located just above the applicator or pressure regulator.

Copy of this presentation will be posted at